1. Field of the Invention
This invention relates to frequency mixing of signals and, more particularly, to a frequency mixer with an extended dynamic range which can be used for the frequency upconversion and/or downconversion of signals.
2. Description of Related Art
Frequency conversion of signals is primarily accomplished by a frequency mixer element. The frequency mixer multiplies two or more input signals in the time domain or convolves one or more input signals in the frequency domain. For example, for certain frequency conversion applications, the frequency mixer mixes an input signal having a frequency f1 and a local oscillator signal having a frequency f2. By mixing these signals, the mixer produces first order frequency converted signal components having the frequencies f1+f2 and |f1xe2x88x92f2| with the amplitude or shape characteristics of the input signal. If frequency upconversion is desired, the lower frequency signal component is filtered out to leave an upconverted signal, and if frequency downconversion is desired, the higher frequency signal component is filtered out to leave a downconverted signal.
The mixing of the input signal and the local oscillator signal, however, also generates intermodulation distortion. In general, intermodulation distortion results from spurious combination frequency components in the output of a nonlinear element when two or more sinusoidal signals form the input. Intermodulation distortion of a complex wave (having multiple frequency components) arises from intermodulation of the components in the complex wave by each other in a nonlinear system, producing waves having frequencies, among others, equal to the sums and differences of the components of the original wave. The power level of the intermodulation distortion generated by a mixer depends upon the input signal power level. Typically, for an increase in input signal power level, the mixer generates a corresponding increase in output signal power level with an even greater increase in the power level of the intermodulation distortion. As such, the highest acceptable power level of intermodulation distortion resulting from the corresponding highest output signal power level generally defines a boundary for the dynamic range of the mixer. The dynamic range of the mixer can be defined for a given output power level as the difference between the output signal power level and the corresponding power level of the intermodulation distortion. Whether the dynamic range is acceptable depends on the particular application. If a given output signal power level exceeds the dynamic range of the mixer, this usually means that an unacceptable power level of intermodulation distortion is generated by the mixer along with the frequency mixed or converted output signal. Extending the dynamic range allows the mixer to produce a greater range of output signal power levels without generating unacceptable levels of intermodulation distortion. For example, in an application where a mixer is operating in a 30 kHz bandwidth, a mixer can have a dynamic range of 100 dB defined by a high output signal amplitude of 0 dBm and a corresponding intermodulation distortion amplitude of xe2x88x92100 dBm. Extending the dynamic range of the mixer occurs by increasing the relative difference between the amplitudes of the output signal and the intermodulation distortion.
A frequency mixer with an extended dynamic range is desirable.
The present invention involves a frequency mixing system that provides an expanded dynamic range when compared to the dynamic range(s) of an individual mixer(s) that makes up the frequency mixing system. The frequency mixing system adjusts the amplitude of an input signal to be frequency mixed to produce a frequency converted signal with an acceptable and/or lower (when compared to the amplitude of intermodulation distortion produced by mixing the input signal without amplitude adjustment) amplitude of intermodulation distortion. If the input signal were frequency mixed without the amplitude adjustment, an unacceptable and/or higher level of intermodulation distortion would result (when compared to the corresponding intermodulation distortion if the amplitude-adjusted signal were mixed by an individual mixer). Adjusting the amplitude of the input signal creates an adjusted signal with signal distortion on the first path. The frequency mixing system uses a feed-forward arrangement to reduce the signal distortion created by adjusting the amplitude of the input signal, thereby producing the desired frequency converted signal with the lower and/or acceptable level of intermodulation distortion. For example, the signal distortion from the first path can be placed on a second path, frequency converted using a second mixer on the second path, and subsequently put back into the first path to combine with the signal distortion on the first path to provide the desired frequency converted signal with the acceptable and/or lower level of intermodulation distortion. By increasing the relative difference between the amplitudes of the desired frequency converted signal and of the intermodulation distortion, the frequency mixing system provides an expanded dynamic range.
In certain embodiments, a limiting device on the first path limits the amplitude of the signal on the first path, thereby producing signal distortion emanating from the limiting device with the signal. The signal on the first path is then frequency mixed along with the signal distortion by a first mixer. The mixing of the signal generates an acceptable and/or lower level of intermodulation distortion because the input signal on the first path is amplitude adjusted or xe2x80x9cclippedxe2x80x9d to produce a converted signal within the dynamic range for the first mixer whereas the input signal without amplitude adjustment would have produced a converted signal outside the dynamic range of the first mixer. To remove the signal distortion from the first path caused by adjusting the amplitude of the signal on the first path, the signal distortion on the first path is isolated on the second path. To isolate the signal distortion on the second path, the signal on the first path along with the signal distortion is coupled onto the second path. The signal coupled from the first path is designed to be about 180 degrees out of phase with the signal on the second path. The signal from the first path combines with the signal on the second path, producing the signal distortion created by adjusting the amplitude of the input signal on the second path. The signal distortion on the second path is then frequency converted by a second mixer. The frequency converted distortion on the second path is coupled to the output of the first path to combine with the frequency converted distortion on the first path, thereby producing the desired frequency converted signal with the acceptable and/or lower level of intermodulation distortion.